Alan G. Davenport Wind Engineering Group
Alan G. Davenport Wind Engineering GroupWIND TUNNEL TESTING:A GENERAL OUTLINEMay 2007The Boundary Layer Wind Tunnel LaboratoryThe University of Western Ontario, Faculty of Engineering ScienceLondon, Ontario, Canada N6A 5B9; Tel: (519) 661-3338; Fax: (519) 661-3339Internet: www.blwtl.uwo.ca; E-mail: info@blwtl.uwo.ca
TABLE OF CONTENTS1 INTRODUCTION12 THE MODELLING OF THE SITE AND THE WIND2.1General2.2Scaling2223 THE CLADDING LOADS TESTS3.1Pressures and Suctions on Exterior Surfaces3.2Scaling3.3Internal Pressures and Differential Pressures33334 THE DETERMINATION OF OVERALL STRUCTURAL LOADS AND RESPONSES4.1Introduction4.2The Force Balance Test4.3The Two-degree-of-freedom Aeroelastic Test4.4The Multi-degree-of-freedom Aeroelastic Test4.5Overall Loads from Local Pressure Measurements4.6Effective Static Force Distribution4.7Load Combination Factors444556675 THE PEDESTRIAN LEVEL WIND SPEED TEST86 THE TESTING OF LONG SPAN BRIDGES97 OTHER TESTS10REFERENCES11APPENDIX AA-1THE DEFINITION OF WIND CLIMATEA.1IntroductionA.2Natural WindA.3Availability of Wind RecordsA.4Probability Distribution of Mean Wind Speed and DirectionA.5Applicability of the Wind Climate ModelA-1A-1A-1A-2A-3A-4APPENDIX BB-1THE DEFINITION OF A HURRICANE WIND CLIMATEB.1IntroductionB.2The Approach UsedB.3Verifying the ApproachB.4The Wind Climate for a Particular SiteB-1B-1B-1B-2B-2APPENDIX CC-1THE MEASUREMENT AND PREDICTION OF SURFACE PRESSUREC.1Experimental TechniqueC.2Experimental Time ScaleC.3Choice of Sampling PeriodC.4Definition of the Pressure CoefficientsC-1C-1C-1C-2C-2WIND TUNNEL TESTING:A GENERAL OUTLINE-i-Alan G. Davenport Wind Engineering Group
C.5C.6C.7General Characteristics of the Pressure ResponsePredictions of Peak Pressures and SuctionsLimitations on the Predicted Peak Pressures and SuctionsC-3C-3C-3APPENDIX DD-1PREDICTING PEAK RESPONSES FOR VARIOUS RETURN PERIODSD.1IntroductionD.2The Prediction ProcessD.3The Rate of Up-crossing of Peak (Maximum or Minimum) Response ValuesD-1D-1D-1D-2APPENDIX EE-1STORM PASSAGE PREDICTIONS OF WIND LOADS AND RESPONSESE.1OverviewE.2Extreme-value Predictions from Time-domain AnalysisE.3Examples of Predictions of Wind Loads and EffectsE.4Wind DirectionalityE-1E-1E-1E-2E-3APPENDIX FF-1DETERMINATION OF INTERNAL PRESSURE COEFFICIENTS AND THE FORMATION OFDIFFERENTIAL PRESSURE COEFFICIENTSF.1SummaryF.2IntroductionF.3Mean Internal Pressures: Distributed LeakageF.4Mean Internal Pressures: Large OpeningsF.5Fluctuating Internal PressuresF.6Forming Differential Pressure Coefficients at BLWTLF.7Limitations of Predicted Differential PressuresF-1F-1F-1F-1F-2F-3F-4F-5APPENDIX GG-1DETERMINATION OF TOTAL DYNAMIC LOADS USING A RIGID MODEL/FORCE ts of the Force MethodG.4Concept of the BalanceG.5Linear Elastic Response Calculations with the BLWT BalanceG.6Torsional ResponseG-1G-1G-1G-1G-2G-2G-4APPENDIX HH-1AEROELASTIC SIMULATIONS OF BUILDINGS USING TWO-DEGREE-OF-FREEDOM MODELSH.1IntroductionH.2Aeroelastic ModellingH.3Details of the Aeroelastic ModelH.4Experimental Procedure and PreliminariesH.5Predictions of Peak Wind-Induced ResponseH-1H-1H-1H-2H-2H-3APPENDIX II-1AEROELASTIC SIMULATIONS OF BUILDINGS USING MULTI-DEGREE-OF-FREEDOM MODELS I-1I.1IntroductionI-1I.2Aeroelastic ModellingI-1WIND TUNNEL TESTING:A GENERAL OUTLINE- ii -Alan G. Davenport Wind Engineering Group
I.3I.4I.5Details of the Aeroelastic ModelExperimental Procedure and PreliminariesPredictions of Peak Wind-induced ResponseI-2I-2I-3APPENDIX JJ-1DETERMINATION OF TOTAL DYNAMIC LOADS FROM THE INTEGRATION OF SIMULTANEOUSLYMEASURED PRESSURESJ-1J.1SummaryJ-1J.2The Integration ProcedureJ-1J.3Response CalculationsJ-1APPENDIX KK-1THE EVALUATION AND USE OF EFFECTIVESTATIC FORCE DISTRIBUTIONSK.1Effective Static Force DistributionsK.2Combined Load CasesK-1K-1K-2APPENDIX LL-1THE PEDESTRIAN LEVEL WIND ENVIRONMENTL.1IntroductionL.2Test ProcedureL.3Statistical Predictions of Pedestrian Level WindsL.4Acceptance and Safety Criteria for Pedestrian Level Wind ConditionsL-1L-1L-1L-1L-2APPENDIX MM-1DYNAMIC WIND FORCES ON LONG SPAN BRIDGES USING EQUIVALENT STATIC LOADSM.1IntroductionM.2The Description of Design LoadsM.3Evaluation of the Modal Load W1 and W2M.4Experimental Determination of Design Load ComponentsM.5Determination of Design Wind LoadsM.6ConclusionsM-1M-1M-1M-1M-4M-4M-5WIND TUNNEL TESTING:A GENERAL OUTLINE- iii -Alan G. Davenport Wind Engineering Group
LIST OF TABLESTABLE L.1CRITERIA FOR PEDESTRIAN COMFORT AND SAFETY.L-3TABLE L.2EXTRACTS FROM THE BEAUFORT SCALE .L-4WIND TUNNEL TESTING:A GENERAL OUTLINE- iv -Alan G. Davenport Wind Engineering Group
LIST OF FIGURESFIGURE B.1COMPARISON OF TYPHOON WIND SPEEDS AT WAGLAN ISLAND TOMEASURED DATA CORRECTED FOR TOPOGRAPHIC EFFECTS . B-5FIGURE B.2PREDICTED PEAK ACCELERATIONS FOR THE ALLIED BANK DURINGHURRICANE ALICIA. B-6FIGURE D.1ILLUSTRATION OF THE PREDICTION PROCESS . D-4FIGURE E.1OBSERVED 10-MINUTE AVERAGE SURFACE WIND SPEED AND WINDDIRECTION AT HONG KONG DURING TYPHOON YORK (SEPTEMBER 16,1999). . E-5FIGURE E.2TYPICAL WIND INDUCED RESPONSE SHAPES. E-5FIGURE E.3COMPARISON OF GENERIC WIND LOADS AND EFFECTS PREDICTEDFOR DIFFERENT WIND DIRECTIONS USING CONVENTIONALSTATISTICAL METHODS AND TRACKING THE EFFECTS OF INDIVIDUALSTORMS. E-6FIGURE E.4COMPARISON OF PEAK STRUCTURAL WIND LOAD EFFECTS FORBUILDINGS A AND B HYPOTHETICALLY LOCATED IN DIFFERENT WINDREGIONS PREDICTED BY CONVENTIONAL STATISTICAL METHODS ANDBY TRACKING THE EFFECTS OF INDIVIDUAL STORMS . E-6FIGURE E.5COMPARISONS OF PREDICTED LOCAL PEAK PRESSURES ANDSUCTIONS FOR A SPECIFIC BUILDING AND FOR A GENERIC PRESSURECOEFFICIENT DATA SET IN DIFFERENT WIND REGIONS USINGCONVENTIONAL STATISTICAL METHODS AND BY TRACKING THEEFFECTS OF INDIVIDUAL STORMS. E-7FIGURE E.6DIRECTIONALITY FACTORS FOR GENERIC PEAK STRUCTURAL LOADSAND RESPONSES USING CONVENTIONAL PREDICTIVE METHODS ANDSTORM PASSAGE TRACKING . E-7FIGURE G.1DYNAMIC RESPONSE OF THE BALANCE-MODEL COMBINATION . G-6FIGURE H.1SCHEMATIC OF THE AEROELASTIC MODEL . H-4FIGURE I.1SCHEMATIC OF THE AEROELASTIC MODEL .I-4FIGURE M.1DISTRIBUTED WIND LOAD COMPONENTS .M-6FIGURE M.2NOTATION .M-6FIGURE M.3SPECTRUM OF MODAL LOAD AMPLITUDE .M-7FIGURE M.4SUNSHINE SKYWAY BRIDGE.M-7FIGURE M.5SECTION MODEL RESPONSE (UNCORRECTED) .M-8FIGURE M.6VERTICAL VELOCITY SPECTRUM.M-9FIGURE M.7AERODYNAMIC ADMITTANCE RESPONSE (UNCORRECTED) .M-9WIND TUNNEL TESTING:A GENERAL OUTLINE-v-Alan G. Davenport Wind Engineering Group
FIGURE M.8JOINT ACCEPTANCE FUNCTION.M-10FIGURE M.9DAMPING FUNCTIONS .M-10FIGURE M.10 WIND LOAD COMPONENTS ON COMPLETED BRIDGE .M-11WIND TUNNEL TESTING:A GENERAL OUTLINE- vi -Alan G. Davenport Wind Engineering Group
1 INTRODUCTIONThis document provides a general outline of common wind tunnel tests performed at the BoundaryLayer Wind Tunnel Laboratory (BLWTL) at the University of Western Ontario. It also details some of thetechniques used to analyse the data from these tests. Since it is a general outline, it will cover some testsand analyses not performed for a particular project. Other than the wind climate modelling discussed inSection 2 and Appendices A and B, and the prediction methodology discussed in Appendices D and E,the various tests and analysis methodology are independent and the reader may skip sections that arenot relevant. Also, this report does not, by any means, attempt to cover all of the types of tests andanalyses performed at the Laboratory. Unusual tests are covered in separate reports for the projectsemploying them.In determining the effects of wind for a particular development, there are two main ingredients toconsider. The first comprises the aerodynamic characteristics of the development. These are simply theeffects of the wind when it blows from various directions. This information only has limited value, however,without knowing how likely it is that the wind will blow from those directions and how strongly it is likely toblow. This climatological information, in the form of a probability distribution of wind speed and direction,is the second main ingredient needed for determining wind effects for a particular development. Theaerodynamic information is characteristic of the particular development and its immediate surroundings,while the wind climate information is characteristic of the geographical location of the development. Bothare necessary to determine the wind effects for a particular development and, when combined, providestatistical predictions of the wind effects which are independent of wind direction.At the BLWTL, the aerodynamic characteristics of the development are commonly determinedthrough model studies of the project. These studies may include measurements of various types ofinformation of interest, such as cladding loads, structural loads and pedestrian level wind speeds, asdetailed in the following sections of this report. The probability distribution of wind speed and direction, isdetermined from analyses of historical wind speed and direction records taken near the site of thedevelopment. Details of these analyses are included in Appendix A. Tropical cyclones, such as hurricaneor typhoon winds present a special case and their associated statistical characteristics are handledseparately using different analysis methods. These are detailed in Appendix B.In all cases, tests carried out at the BLWTL are in accordance with the state-of-the-art, and meet orexceed such test requirements as documented by the ASCE Manual of Practice (1).WIND TUNNEL TESTING:A GENERAL OUTLINE-1-Alan G. Davenport Wind Engineering Group
2 THE MODELLING OF THE SITE AND THE WIND2.1GeneralThe basic tool used is the Laboratory's Boundary Layer Wind Tunnel. This wind tunnel is designedwith a very long test section, which allows extended models of upwind terrain to be placed in front of themodel of the development under test. The modelling is done in more detail close to the site. The windtunnel flow then develops characteristics which are similar to the wind over the terrain approaching theactual site. This methodology has been highly developed and further details can be found in References2, 3 and 4.The modelling is comprised of the following components:1.A detailed model of the development. Different types of model are used for the various types oftest. These are discussed below in the sections on the individual tests.2.A detailed proximity model of the surrounding area, built in block outline from wood andStyrofoam. Depending on the scale and size of the model, this may extend for a radius ofapproximately 500 to 600 metres.3.Coarsely modelled upstream terrains, chosen to represent the general roughness upstream ofthe site for particular wind directions. Typically, several models are chosen, each used for arange of wind directions.For project sites close to hilly terrain or with unusual topography, topographic study may be carried out toestablish the wind characteristics at the site. This may be in the form of topographic model study at asmall scale ( 1:3000) or computational methods. The resulting target wind characteristics will bemodelled in the larger scale used in the building or bridge tests.2.2ScalingThe fundamental concept is that the model of the structure and of the wind should be atapproximately the same scale. The natural scaling of the flow in the wind tunnel is in the range 1:400 to1:600; however, in some cases, instrumentation or other requirements may demand a larger model. Inthese cases, additional flow modification devices may be used to approximate larger scale flows.In all cases, it is the mean wind speed profile and the turbulence characteristics over the structurethat are most important to match with those expected in full scale. Guidance as to the latter is obtainedthrough direct full scale measurements as compiled by ESDU (5, 6). Such data are also used to ensurethat the test speeds near the top of the building are properly interfaced with full scale wind speedspredicted to occur at the full scale site.WIND TUNNEL TESTING:A GENERAL OUTLINE-2-Alan G. Davenport Wind Engineering Group
3 THE CLADDING LOADS TESTS3.1Pressures and Suctions on Exterior SurfacesDetailed measurements of the pressures and suctions on exterior surfaces of the building or structureare made using a rigid model that accurately represents the detailed exterior geometry of thedevelopment. The model contains numerous (typically 300 to 800) holes or "taps" which are connectedvia tubing to pressure transducers. The transducers convert the pressure at the point where the tap islocated to an electrical signal which is then measured by the Laboratory's computerized data acquisitionsystem. The technology employed allows all pressures on the building to be measured essentiallysimultaneously for a particular wind direction. Measurements are usually made at 10 intervals for the full360 azimuth range. A detailed description of the procedures followed and the definitions used arepresented in Appendix C.These aerodynamic measurements made in the wind tunnel are subsequently combined with thestatistics of the full scale wind climate at the site using the methodology outlined in Appendices D and Eto provide predictions of pressures and suctions for various return periods.3.2ScalingThe aerodynamic pressure coefficients can be converted to full scale pressure values based onconsistent length, time and velocity scaling between full scale and model scale. This applies very well forsharp-edged structures. For structures with curved surfaces, additional care has to be taken to ensurethat the flow regime is consistent in model and full scale, as well as in the interpretation of the results.For typical building tests, length scale is in the order of 1:300 to 1:500. Velocity scale is approximately1:3 to 1:5. Time scale is in the order of 1:100. For example, 36 seconds in model scale represents aboutan hour in full scale and the data will be taken about 100 faster in the test than in full scale. Further detailsregarding scaling can be found in Appendix C.3.3Internal Pressures and Differential PressuresThe net load on cladding is the difference between the external and internal pressures. Using themethodology described in Appendix F, mean internal pressures are determined at all wind angles. Theseare then subtracted from the appropriate external pressure coefficients to form differential pressurecoefficients. Finally, the coefficients are combined with the statistics of the full scale wind climate at thesite, using the methodology outlined in Appendices D and E, to provide predictions of differentialpressures and suctions for various return periods.In the case of large opening due to operable windows or breach of the building envelope, largeinternal pressures may develop. Typically, the external pressure at the opening will be transmitted into thebuilding interior volume. Building envelope at other locations within the building volume will experienceboth the external pressures at those external locations as well as the large internal pressure transmittedfrom the opening.For free standing elements with both sides exposed to air, such as parapets and canopies, the netdifferential pressures are the instantaneous difference in pressures on the opposite sides.WIND TUNNEL TESTING:A GENERAL OUTLINE-3-Alan G. Davenport Wind Engineering Group
4 THE DETERMINATION OF OVERALL STRUCTURAL LOADS ANDRESPONSES4.1IntroductionThe dynamic response of most tall buildings to wind is primarily the results of building motions in thefundamental sway and torsion modes of vibration with relatively small contributions from higher modes.The mechanical transfer function, relating the load function to the response, is straightforward. On theother hand, the aerodynamic transfer function, relating the gust structure to the wind induced forces isdifficult to establish without wind tunnel model tests. A further complication exists if body motion effectsinteract with the load function (aerodynamic damping).Multi-degree-of-freedom aeroelastic models have traditionally been used to study the action of windon sensitive buildings and structures. While such simulations provide the most direct and reliableestimates, the required models are expensive and time consuming to design and construct. Two-degreeof-freedom aeroelastic models, which simulate the wind induced responses in the two fundamental swaymodes of vibration, while less expensive, do not provide information on torsion effects, which may besignificant for buildings of unusual shape and structural dynamic properties. In both of these cases, themodel moves in the wind tunnel just as it would in full scale; its response in the wind tunnel can be scaleddirectly to full scale.A high-frequency balance/model system can measure the load function directly, provided thataerodynamic damping effects are negligible, which is usual for most buildings at practical wind speedsand practical structural damping values. (Note that if such effects are important, they can be accountedfor by using a supplemental testing technique in which the model is oscillated). The now commonly usedhigh frequency force balance technique was originally developed at the BLWTL.One other method for determining the load function is to integrate the point pressure measurementson an instant-by-instant basis to form time histories of the generalized forces.4.2The Force Balance TestThis technique involves testing a lightweight, stiff, geometrical representation of the building on anultra-sensitive force balance. The technique allows direct measurements of good approximations to thesteady and unsteady modal forces acting in the fundamental sway and torsional modes of vibration of thebuilding. The dynamic responses including resonant amplification at the natural frequencies of thebuilding are derived analytically for each mode using random vibration analysis methods and aresubsequently used to provide estimates of the full scale responses of the building. As a result, thismethod is very accommodating of changes to the structural properties after testing, since the analyticalprocedure can be simply repeated using the same experimental results, which remain applicable so longas the aerodynamic characteristics of the building remains the same. A detailed description of thismethod is presented in Appendix G.Time histories of the base she
WIND TUNNEL TESTING: - 2 - Alan G. Davenport Wind Engineering Group A GENERAL OUTLINE 2 THE MODELLING OF THE SITE AND THE WIND 2.1 General The basic tool used is the Laboratory's Boundary Layer Wind Tunnel. This wind tunnel is designed with a very long test section, which allows exten
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